The present invention is directed to position determining hardware. More particularly the present invention is directed to an adaptive multi-axis sensor array known as Ensemble of Quartz Clocks Adapting To the Environment (EQUATE).
Systems capable of providing position, acceleration and velocity data are well known in the prior art. The more common ones utilize signals provided by a Global Position System (GPS) which are fed to an interface with a navigation computer to provide position, velocity and time reference signals. Other systems employ microwave signals to provide a localized solution as for an airport. Radio transmitters are used in certain applications to triangulate to provide independent navigation data.
Existing position detecting systems such as satellite based systems including GPS, VOR, cell tower signals, other microwave signals, have significant limitations. Although these systems provide useful information for vehicular navigation (both earthbound and space/airborne), communications links, geodesy and mapping, to name a few, each has its problems. Some areas have limited/no access to microwave signals. Further, such signals are subject to jamming, spoofing, or otherwise degraded, such that significant distortion in accurate positioning determination is created. Each of the existing systems require constant data monitoring and updating to work properly. Some solutions propose combining several of these existing systems in an attempt to remedy the short-comings of each. However, these attempts to provide a suitable “fail-safe” system are doomed to failure if updated signal information is unavailable for whatever reason. Further, current applications including deep sea exploration, navigation in polar icecap regions with only minimal satellite GPS data available, determining position in caves or buildings which are not microwave penetrable, deep oil exploration, precisely locating orbiting spacecraft, celestial objects, or objects on the dark side of the moon, find no solution in existing systems. Most of these systems do not work well in “urban canyons” due to signal blockage and multi-path problems.
Advanced navigational systems currently available employ Kalman filters or some variation such as an extended or adaptive Kalman filter, to improve the accuracy of the output from the positioning device. This is required since the signal acquisition device acquires both the signal and some degree of “noise”. Kalman filters and their variants, are adaptive, i.e., they modify the initial recorded data to adjust for the signal noise. Kalman filters, which typically interpolate or extrapolate to provide a suitable estimate for error correction, are designed to operate most effectively in correcting for linear variations in a continuous data stream. If the errors require the system to accommodate a non-linear estimation, involve a discontinuous data stream, or noise that involves non-stationary modeling, the effectiveness of the Kalman filter based system is significantly hampered/limited.
Some hybrid systems involving inertial measurements are capable of overcoming short-term errors in receiving microwave signals. However, it is critical in these systems that the data streams from the GPS and the data stream from the Inertial Measurement Units (IMU) be precisely aligned with respect to time. To accomplish this, the time reference signal must be of the highest precision. The signals from the available systems are of insufficient precision.
It is among the objects of the present invention to overcome the limitations and deficiencies of the prior art systems. Particularly, the signal provided by the EQUATE oscillator ensemble provides a signal of the necessary precision enabling the tight synchronization needed for the two data streams. In addition, the EQUATE oscillator array uses an Allan filter which is capable of providing accurate signal correction even when non-linear corrections are necessary and even though non-continuous external data is available, and even though the noise my be non-stationary in nature. What is more, the EQUATE array can provide precision position signals when no external data stream is available.
The EQUATE array comprises a plurality of 2n piezoelectric oscillators arrayed in pairs, where n is the number of axes of interest, and each member of a respective pair has its principal operational axis disposed opposite that of its associated oscillator. Each oscillator has a resonator and ancillary circuitry and is subject to both i) environmental effects including acceleration, mechanical shock, temperature variations, and vibration; and ii) systematic effects including aging, frequency drift, and time offset. Controller means is provided which employs dynamic relationships between the plurality of oscillators in the ensemble to determine a magnitude of correction needed, and to produce estimated weighting factors for each oscillator to correct for both the environmental and the systematic effects. Preferably, the controller means includes a microprocessor and a signal conditioner which transforms input and output signals into a digital readout. The oscillators are also, preferably, quartz crystals or other material having similar anisotropic characterization properties.
Most preferably, the ensemble is configured as 3 pairs of opposed oscillators oriented along three orthogonal axes to cover all possible axial and rotational axes of interest. At least one of the quartz oscillators, and preferably each, is a stress compensated (SC) cut resonator operating in dual mode, C mode for frequency output, and B mode for temperature compensation. This oscillator array will be embedded in an application specific integrated circuit (ASIC) to reduce spacial requirements and power requirements. Specifically, embedding the ensemble in an ASIC reduces total occupied space to less than 0.8 cc and power requirements to 15 mW. In one preferred embodiment, a seventh oscillator canted by 45° to each of the x, y, and z axes is provided to supply a reference signal, as well as a consistency check on the acceleration readings obtained from the other three pairs. Alternatively, a reference signal may be supplied by an external source. EQUATE is robust, being capable of sensing statistically significant errors and not be perturbed by their effects. The oscillator array is provided with a housing of rugged construction enabling it to withstand a temperature range varying between −55° C. and 300° C. without detrimental effect on signal output and withstanding the vacuum of deep space, the high pressure and temperature of well bore conditions, and the high pressure and humidity of oceanographic terrain.
The sensor array for EQUATE overcomes the problems of double integration of errors common to accelerometry in determining position (i.e., integration of acceleration to determine velocity, velocity to determine position; an error in the acceleration data is magnified with each integration). The theoretical error-of-position dispersion rate for this technique is a function of τ1/2 and a feasibility study has determined that the actual dispersion rate is 0.7×τ1/2 mm where r is time measured in seconds. EQUATE array achieves exceptional accuracy over extended periods with the error only growing to a distance only slightly greater than 1 meter over an entire month, better than the accuracy performance of GPS.
The EQUATE array has been specifically designed for low power consumption such that it can be easily incorporated into handsets, satellites, and the like. The algorithm will compensate for shock, vibration, and acceleration perturbations associated with its transport. It can operate over large ambient temperature ranges (including military temperature ranges of −40° C. to 125° C.) without the need for an oven. Further, the sensor array automatically compensates for effects typically occurring in quartz-crystal oscillators as a result of temperature changes.
The EQUATE sensor array is unlike other systems whose accuracy degrades with time. EQUATE's algorithm uses a statistical theorem for whiteness to optimize the estimates of the temperature coefficients unique to each sensor ensemble. It adaptively learns and updates its acceleration sensitivity coefficients so that it improves its performance with time. So, in contrast to most clock technologies that necessarily attempt to shield their systems from environmental effects of temperature, shock, vibration, and acceleration, EQUATE uses these environmental effects to upgrade parameter sensitivity estimates. By continually upgrading these parameters, EQUATE's environmental immunity actually improves with time turning the environmental effects harmful to other systems, into tools for enhancing performance.
The time-ensemble control methodology, including algorithms, employed in the EQUATE sensor array has been proven in oscillator hardware at Oak Ridge National Laboratory (ORNL) to provide optimum timekeeping performance ensuring superior output than the best clock utilized in the EQUATE ensemble. Further, this methodology enhances the performance of even the worst clock and can better deal with measurement noise and flicker noise than existing Kalman approaches.
EQUATE's electronics can be optimized by using the latest ultra-low noise SiGe devices and circuitry and could be nicely implemented with custom ORNL-designed mixed-signal SiGe integrated circuits to provide absolutely the lowest noise, power, temperature/shock compensation available in the smallest package.
The EQUATE sensor array can be steered using an external reference, such as GPS or even advanced common-view GPS synchronization algorithms, which can utilize the high precision of the EQUATE technology. This information also provides a calibration reference from auxiliary sensor, should GPS be unavailable for any reason. EQUATE can “flywheel” the latest values forward with very high precision and low dispersion of error estimates.
The EQUATE algorithms allow for the measurement of and removal of typical systematic effects plaguing quartz-crystal oscillators, such as time offset, frequency offset, and frequency drift, for example. The updating of the coefficients for removal of these systematic effects can be continuous or intermittent.
Unlike most accelerometers, the EQUATE sensor array can sense both acceleration effects as well as instantaneous velocity. This is due to the linear relationship between acceleration and velocity. By measuring phase directly, the instantaneous velocity can be determined. By eliminating the step of integrating frequency/acceleration to obtain velocity, EQUATE avoids the long-term performance degradation associated with integration of errors. EQUATE can provide measurements of translational and rotational motion. In addition, EQUATE can measure local vertical direction. In performing this measurement, it uses the earth's gravitational field as a calibration reference. Feasibility studies have verified accuracies of these measurements with errors of not greater than 0.4°. If the EQUATE sensor array is used with a magnetometer or similar auxiliary signal, it can provide measurements of orientation. EQUATE has the ability to integrate out short-term magnetometer fluctuations and ascertain anomalous magnetic environmental perturbations.
With respect to timing, EQUATE is, to a large degree, self-calibrating, knowing the performance of each of the sensors in the ensemble and, hence, of the entire EQUATE ensemble. As a result, the timing error dispersion rate for EQUATE is minimized and can be estimated from the ensemble elements.
With regard to position, EQUATE is self-calibrating, measuring the effects of low frequency dispersive processes common to clocks and oscillators, such as white noise, flicker noise, and random-walk noise, and removing, to a large degree, the effects these dispersive processes would otherwise cause. Accordingly, EQUATE's position error performance for a small, low-powered device is unsurpassed when operating independently of external signals, both in the short-term and long-term operations.
The EQUATE sensor array is unique in that once it has fiducial reference points in space and time, it can flywheel both the time and the position, which results in a continuous estimate of velocity, acceleration, and orientation. The local vertical is determined independently from a fiducial reference.
The pairing of the six oscillators in the preferred embodiment enable the EQUATE sensor array to respond maximally to forces in the ±x, ±y, and ±z directions. Reversing the sensitivity directions along a single axis allows the measurement of approximately double the frequency change as a force along that particular axis causes the two paired oscillators to be offset in opposite directions. These changes can be measured singly against the more stable ensemble frequency reference for individual sensitivity calibration, or compared against each other to measure the total change in frequency due to fore in the sensitivity direction.
The orthogonal orientation of the sensitivity axes of the three pairs of oscillators allows measurement of torque forces during rotation about any axis. This can be distinguished from translational forces, as the torque forces are either radially inward or outward. This causes the frequency changes in opposing oscillators which have a moment around the rotational axis to both move in the same direction (both positive or both negative).
At least one, and possibly more, additional oscillator(s) (i.e., reference oscillator) can be placed to have equal sensitivity to +x/+y/+z and −x/−y/−z as a measure of consistency. The algorithms provide a constant health status of the EQUATE sensor array, making the system extremely robust and reliable.
Various other features, advantages, and characteristics of the present invention will become apparent after a reading of the following detailed description.